Catamaran Conversion

The figures you quote are differnt from their website, says 22,800lbs for such figures, not the 29,900lbs! (from what i could find in the short scan anyway)..so that is a clue not all is correct.

If you look at his figure of fuel consumption, this gives a g/kWh for the D3-160 engine of some 240g/kWh. It is this that you need to find the context. How does this engine compare with others. Then you can address the hull part. Since if you install a thristy engine, doesn't matter what you hull is doing, the engine shall always be drinking! The hull will just make it worse if a poor hull.

So, in the 160hp range what are your options and their fuel consumption figures?..how do they compare?

Quick scan gives you this:
http://www.volspec.co.uk/Products/Marine_Commercial_Engines/D3-190DPS.pdf
The Volvo at some 34l/h
and a Yanmar:
http://www.yanmarmarine.co.uk/pdfs/data/4BY150.pdf
at 30l/h
or the perkins-sabre:
http://www.perkins-sabre.com/Engines/M150Ti/Performance.cfm
33l/h
or the john deer:
http://www.deere.com/en_US/rg/ESC/SpecSheet/GenSet/6068TF150_C_S0_R0.pdf
at 37l/h

So, which would you choose, based upon fuel consumption?

Im used to working with engines in the 200-210 g/kWh range.

It is all relative.

 

You're referring to the MaineCat? Their claimed full-load displacement is 29,900 lbs. Here http://www.mecat.com/power/powerdesign.htm and several other places on their website.

I do not know what displacement they tested at, and that's a fair question. Their hulls are new so I'm not familiar with much in the way of independent testing.

I would be considering the Volvo D3 or D4 engines initially, in part because I could hopefully then re-use much of the Volvo stuff I currently have installed (gauges, controls, NMEA 200 bridge, etc.). The size/power of the engines will largely be determined by the demands of the hull at the desired speed, and which engine's torque curve matches up the best.

 

Yes, their performance figure state done at 22,800lbs:
http://www.mecat.com/power/powerupdates.htm
Later in the blog, they state this to be 11.4 ton, which is a short ton, and a nonsense unit (22800lbs = 10.34 tonne) just trying to sound heavier than it really is..

for the P47 which would have an L/D of what..6.3 on those figures, very different from yours!

Thus you need to be carfeul to compare apples with apples. sales stuff is always misleading..for obvious reasons.

If in doubt see attached:



You can see the effect of L/D ratio very easily....yours at 4.7-ish and the P47 at 6.3. Don't need to be a rocket scientist to see why when looking at the curves.

 

Apples to apples indeed. And with the hull modifications I've proposed, my L/D would be around 5.8.

 

Deering,

I am by no means a naval architect but i have been sailing catamarans since i was a child and worked at sea for many years on all sorts of boats... i offer my opinion and reasoning to your dilemma as follows;

To put it simply, fix the volvo outdrive as per the original spec and then sell the entire boat for the best price possible... use the funds to start again and build a proper efficient displacement boat or buy another lightly used of the good designs out there.

Why? Because the hull you have will never be as efficient as you would like it to be. Its not just because of the stern shape either. More importantly its because its too heavy, the hulls are too fat (read poor length/beam ratio), and the hulls are not the right shape for efficient displacement style cruising. Simply adding a bulb and lengthening the transoms does not address the key problems with getting your cat to cruise efficiently. So you will end up spending heaps of time and money and only improving the ecconomy slightly from where it is now @ 12kts - not to mention you wont be able to efficiently cruise fast in the 15-20kts range like a good boat will.

Moreover, as this is your retirement and you realise fuel prices are only going upward and youll need an income to run a thirsty boat... everyone else realizes this too so the value of your thirsty boat is going down as fuel goes up, so id sell it while you can as when fuel hits $10 per gallon noone will want to own it - the rich guys will be buying 80ft cats not 40fters...

If you decide to go down this route, make sure the new boat is LIGHT... cats must be light or they dont offer the performance advantages you expect from a cat and being able to cruise in the high teens with low fuel consumption. Look for displacement cats with a lightship weight of around 11-12000lbs in the 40ft class, and slender hulls (high length/beam ratio)... these are the ones that really perform efficiently and fast with low fuel consumption, most all of them are GRP/FRP foam core construction...

 

Believe me, Groper, I've been thinking about that option. What you say makes sense. But...

1. I can probably do a lot more with this hull than just add hull extensions for much less than a new, comparable power cat would cost me. For example, the base price for a MaineCat is around $750K. The delta between my boat and a new highly efficient one would be more than my budget could tolerate.

2. While it'd be nice to cruise at 20 kts, it's not a requirement. 13 kts at decent efficiency would be acceptable.

3. A light weight hull made of GRP and foam will not handle the usage which I put it to. I'm not just riding from dock to dock, using this as a platform for sipping cocktails. I use it as a semi-commercial fishing boat, hauling king crab pots, shrimp pots, even long lines, as well as lots of sport fishing where I'm commonly am beating 150 lbs halibut to death on my deck. I often haul deck loads to remote places as well. Sooner or later, probably sooner, I'd bust a hole through the skin of the superlight hull. I could go with GRP, but to be durable it would end up being more heavy than my aluminum hull.

4. I've spent a lot of time and money configuring this boat to exactly my needs. Moving efficiently and safely through the water is one important role for a boat, but operating effectively once you get there is another. On that latter point, this boat is an all-star.

5. I may have to do more to the hull than just add extensions. One of the beauties of welded aluminum is how much flexibility it allows in modifications. I'm ready to do that if my budget allows. If I do, the weight of this boat will be in line with other power cats that are performing well.

Thanks for your clear-eyed suggestion. On the surface it makes a lot of sense. And it may be the most sensible thing at the end of the day. But really, since when is recreational boat ownership a rational decision in the first place?

 

lightweight cats can and are built in aluminium, they must be engineered thoughtfully tho always with lightweight in mind as are composite boats... its just the norm to see GRP these days...

well if you must persist with this current hull, lengthen the transoms - that duckboard is a waste of what could be waterline length as per ADHOC suggestion... i think he feels that the hull is too beamy to accept a canoe stern, if the converging angles get to great then it will increase drag - he suggests no more than 5 degree vertical run as sqat and negative lift becomes a problem with steep vertical stern runs also... i think a partial canoe stern might be the answer? It depends on how gradually you can fair the converging angles of the canoe... whats the LWL beam of a single hull?

In the Bow, try to improve the entrance angle... your bows reach full width very quickly and so are quite blunt with large entrance angle... extend the waterline at the bow as far forward as you possibly can so the waterline sees a fine entry, this helps efficiency also...

 

No, the L/D ratio is far too low.

 

ok i wont pretend to understand how that works for canoe sterns, but yes, for many reasons your boat would do much better if you could cull some weight from it...

 

Interesting subject thread....wondering if you every came to a decision as to what modification route you might chose?

If I remember correctly there was a gentleman over in Thailand that did a relatively simple version of a prop shaft right out the stern of his cat. I'll see if I can get him to respond to your subject thread and contribute some photos. Frosty was his name, now that I recall.

 

not to jack this thread, but my exp. with Saab&Volvo cars is.....

engines are rock solid, perfect, couldn't ask for better.

Transmissions? Achilles Heel of the vehicle. Not that they don't "do ok", they aren't the "take it for granted" that other overbuilt(?) transmission/gearboxes are.

They just seem to have a higher Engine/Tranny robustness ratio than other manufactures. However, of course, "Your mileage may vary".

I've also noticed that Saab and Volvo(mostly Saab) gearboxes(5spd) shift really slick, so maybe you think you're a better driver than you are, but the unit is taking a beating...but you're having fun.

In my experience, a typical car owner will tell you about how often he changes the engine oil, plugs, air cleaner etc. but will give you a blank look when asked about either an A/T or manual gearbox.


PS-their replacement part prices for transmission/gearbox are very high for engine size.

 

Chain Drive

I think it is foolish to try and put that much HP thru the very small contact areas of bevel gears acting 90 degrees to one another,...and partcularly when it is very torquey diesel power.

Here is what i think is needed, but the one manfacturer who embraced it got back out;
Chain Drive

http://www.yachtforums.com/forums/technical-discussion/2232-new-drive-system-volvo-penta.html#post15453

http://www.yachtforums.com/forums/technical-discussion/13566-power-drive-systems-smaller-planning-hulls.html#post103529

http://www.boatdesign.net/forums/inboards/belt-drive-37290-3.html#post453137

http://www.boatdesign.net/forums/inboards/belt-drive-37290-3.html#post453265

 

A common mistake made by everyone from manufacturers to consumers, is rating driveline components by horsepower (which is irrelevant) when really the torque is what matters with regard to driveline strength and longevity... be nice to see the industry change in this respect...

 

I may have missed it but. Has anybody mentioned that your boat is too heavy for your drives? I considered putting ODs on a 30,000lb boat and the manufaturers pointed out that 11,000lb per unit was at upper limit of units. More recently newer HD units have come out with a 15,000lb upper limit per OD. With your stated weight you probably where way over the limit. That may have something to do with your failure. Poor design by designer or builder.

 

A little hard to fathom how an arbitrary weight limit could be applied and make complete sense, I wonder if it based on the likely efficiency of the set-up rather than any over-stressing, which can surely be accomodated by prop selection ?